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Rapid Natural AttenuationMargaret Allan, M.Eng., P.Eng., P.Geo., EP(CEA)Chris Pooley, B.Sc., P.Geoph.

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Natural Attenuation

volatilization

dispersion

dilution

biodegradation

adsorption

Reduction in mass or concentration of a compound in groundwater over time or distance from the source of constituents of concern due to naturally occurring physical, chemical, and biological processes

—ASTM 2003

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Before and After

2009

2012

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Environmental Process for Site Closure

Assessment

• Phase 1

• Phase 2

Corrective Action

• Remediation/Risk Management

• Reclamation

Regulatory Closure

• Reclamation Certificate Application

• Regulatory Approval

When the river changed course, it removed the soil that had been earmarked for corrective action.

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Chronology

2008 – Well abandoned

2009 – Phase 1 & Phase 2 ESAs

2011 – Floods cause river to change course (spring); exposed flow lines removed (fall)

2012 – Geophysical (fall) and geo-environmental (winter) assessments

2013 – Modeling and reporting

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Impacts Identified in 2009

Area of Site

Contaminants of Concern

Depth

North F2, F3 0-0.15 m

Wellhead E, F1, F3 1-4* m

Process B, E, F1 1-4* m

SumpB, E, F1, F2, F3, Ba

1-3 m

*maximum depth of investigation

Benzene, ethylbenzene, barium and petroleum hydrocarbon fractions F1, F2 and F3 were identified at concentrations exceeding Tier 1 guidelines.

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January 2012

So… Where Did the Contamination Go?

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Mass onsite(kg)

Max conc. in river (mg/L)

Tier 1 FAL/wildlife (mg/L)

Benzene 0.85 0.0002 0.14

Ethylbenzene 10.52 0.002 41

F1 C6-C10* 1363 0.32 9.8

F2 C>10-C16 3753 0.88 1.3

F3 C>16-C34 1929 0.45 ---

Barium 750 0.18 ---

*excluding BTEX

Worst-Case Concentrations in River

Assuming washout in 24 hours and complete displacement into soil-water column, in-river concentrations were estimated:

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Site Status After River Realignment

What went down the river wasnot a cause for concern

from a contaminant standpoint

Unknown how much contamination remains at the well site

• Differences in topography pre/post flood

• How deep is the gravel that was not there before?

• How much original (contaminated) clay till is beneath the gravel?

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Preliminary Conceptual Site Model

Receptors:

• river (FAL)

• water wells

• livestock

• wildlifeCLAY TILL

CONTAMINATION

GRAVEL

• Previously the site was characterized by PHC and Ba contamination in clay till.

• The lowered ground elevation and gravel at surface indicated that the river had washed away some quantity of clay and laid down some quantity of gravel.

• Contamination could still exist beneath the gravel.

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Geophysical and Geo-Environmental Site Assessment Activities, 2012

GEOPHYSICAL ASSESSMENT

• Refraction seismic survey, September 2012

• Different strata exhibit different seismic velocities

• Thickness of sand/gravel; depth to fine-grained soil?

Reference: http://geo-image.com/wp-content/uploads/2014/02/MASW-Hit.jpg

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Geophysical and Geo-Environmental Site Assessment Activities, 2012

GEO-ENVIRONMENTAL

• Drilling/sampling program, December 2012

• Logged stratigraphy for correlation with geophysical results

• Drilled at locations previously deemed contaminated

• Sampled soil for previous contaminants of concern

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Geophysical Results

• Velocities suggest unsaturated gravel, saturated gravel, fine-grained soil

• Gravel as thick as 3 m gave some insight into the amount of scouring (we knew from changed topography that up to 1.8 m of surficial soil had been removed…)

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Geo-Environmental Results

• No remaining evidence of contamination

• Previous consultant had logged clay to 4m

• Beneath the gravel we found silty sand at most places and only a small amount of clay near well centre

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Modeling

Data Sources:

• 2009 LiDAR elevations

• 2009 drilling logs

• 2012 geophysical data

• 2012 surveyed elevations

• 2012 drilling logs

• Understanding of river morphology for aligning data sets

Deliverables Created:

• 3-D model of area investigated

• Stratigraphic cross-sections through previously contaminated areas

• Contour map of difference in ground surface elevation 2009-2012

• Contour map of total soil displaced (2009 ground elevation to base of gravel)

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Review of model and findings

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Summary

• Geophysical and geo-environmental data were acquired and interpreted within a 3-dimensional model

• The model revealed that the river scoured away 53,000 m³ clay till and deposited 34,000 m³ gravel/sand

• Remaining soil met Tier 1 agricultural criteria

• A remediation program on this scale would have cost ~$4million; “rapid natural attenuation” achieved at no cost

• Site was fast-tracked to reclamation certification

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